Conventionally, the bicycle handlebar stem is housed telescopically inside the fork stem and is fastened to the latter by means of a bolt of considerable length, which has a conical nut corresponding to the lower neck of the handlebar stem and which goes through the upper end of the stem where it has an operating head. Under these conditions, when the bolt is turned there is a movement of the conical nut that deforms the lower neck of the handlebar stem causing it to be tightened against the inside wall of the fork stem.
In racing bicycles, in which it is desirable to reduce weight to a minimum, the use of the fastening bolt is not worthwhile as this bolt must have a considerable length, slightly greater than that of the handlebar stem and also a considerable cross-section to support the stresses to which it must be subjected, which results in considerable weight. It has also been proposed to fasten the fork stem to the handlebar stem with a biconical washer that is placed between the upper race housing of the steering, and a stem tightening counternut that is also screwed on the fork stem. Further, the fork stem in its upper end has four cuts in the direction of the generatrices end of considerable amplitude, intended to permit deformations of the stem for its tightening against the handlebar stem, when tightening of the biconical washer is caused by the counternut.
Obviously, the cuts made in the fork stem cause considerable weakening of it so that, even though there is provided at the bottom of the cuts a circular perforation with a diameter slightly larger than the width of the cut, it is obvious that these perforations are not sufficient to prevent such weakening, as any hardened metal that has a structural crack and has to support load and bending with constant variations caused by impacts and vibrations, is capable of breaks extending said cut.
The device for simultaneous tightening of the bicycle steering unit and handlebar stem according to the invention provides maximum guarantees of reliability in tightening and fastening and eliminates the cuts in the fork stem, whereby the strength of the bicycle is increased.